1. Field of the Invention
This invention relates to compound particles of boron (B) and at least one element (X) selected from Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sr and Ca (hereinafter often “hexaboride particles”), and a hexaboride particle dispersion in which the hexaboride particles stand dispersed in a liquid medium or a solid medium (thus, the “dispersion” termed in the present invention refers to a disperse system such as a fluid dispersion or a solid dispersion). More particularly, this invention relates to hexaboride particles improved in water resistance and a dispersion of such hexaboride particles, and also to an article making use of the hexaboride particles or this dispersion.
2. Description of the Related Art
Hexaboride particles typified by LaB6 and so forth have properties such that they have a high light transmittance and a low reflectance in the visible-light region and have a low light transmittance in the near infrared region. Accordingly, in recent years, they are used as solar radiation shielding materials (see Japanese Patent Application Laid-open No. 2000-169765).
Now, the hexaboride particles are known to be surface-decomposed by moisture or water in the air. In particular, where they are present in the state of fine particles, they have increased in their surface area in respect to volume, and hence the surfaces of hexaboride particles are decomposed by the moisture or water to change into compounds of oxides or hydroxides in a large proportion, so that a phenomenon may appear in which the properties inherent in the hexaboride become gradually low.
Then, where coating films or the like making use of hexaboride particles are used for such a purpose of shielding the light in the near infrared region, utilizing their optical properties, the light transmittance in the region of 200 nm to 2,600 nm may rise under the influence of the moisture or water to cause gradual deterioration of solar radiation shielding performance. There has been such a problem, but any methods for preventing it have not been developed.